Microfluidic flow focusing devices have been used to synthesize micrometer-scale emulsion droplets via the thread formation mode of drop breakup. Thread formation occurs when surfactants are dissolved in one or both liquid phases. In a particular range of surfactant concentrations and flow rates, a thin thread is drawn between two primary droplets. As the thread elongates, it disintegrates into a stream of tiny droplets, whose sizes depend on the final thread diameter and the physical properties of the liquids. In the present work, we investigate the role of physical parameters on thread formation. In particular, we show that the thread length is extended when the downstream geometry is modified to stretch the thread more rapidly. In addition, we construct a phase diagram indicating that as the viscosity ratio decreases, the range of capillary numbers in which threads form increases and the threads become longer and thinner. These results suggest ways of optimizing the thread formation process to form ever smaller droplets.